To understand one of the important characteristics of Beijing particulate matter pollution, the mass size distribution of particulate matters and its water-soluble ions, the particle samples were collected by a MOUDI cascade impactor in Beijing during July 2001, March 2002, July 2002 and January 2003 and every sampling period lasted a week. By analysis, the results indicate that there is “two modes" in the diameter range of the fine particle, one called “condensation mode" and another called “droplet mode", and the reasons of the peak of the accumulation mode appearing in the diameter range of 1～1.8μm was discussed in this paper. And there was a relatively strong correlation between PM- 1.8 and PM- 10(R~280%), the same to PM- (1.8～10) (PM- 10 minus PM- 1.8); the ratio of PM- 1.8/PM- 10 varietied between 40% and 60% during the sampling periods; as a result, the mass concentration of PM- 10 can be decreased by decreasing the concentration of coarse particles; sulfate, nitrate and ammonium are the main component of the inorganic water-soluble ions and the percentage of these three ions in PM- 1.8 was above 70% when the concentration of fine particle was more than 70μg·m~ -3 and they are the main reason increasing particle concentration.